Cathode ray tube screen structure



Oct. 29, 1968 ATSUSH] SATO ET AL 3,408,227

CATHODE RAY TUBE SCREEN STRUCTURE =0 6 v 9 l 7 l h C r a M d e l 1 F In UEHTORS HCsushi Sate ysmudi ATTORNEY United States Patent 3,408,227 CATHODE RAY TUBE SCREEN STRUCTURE Atsushi Sato, Kokuhunji-shi, and Masalratu Yamauchi, Musashino-shi, Japan, assignors to Hitachi, Ltd., Tokyo, Japan Filed Mar. 17, 1965, Ser. No. 440,537 Claims priority, application Japan, Mar. 17, 1954, 39/ 14,575 9 Claims. (Cl. 117-212) ABSTRACT OF THE DISCLOSURE A cathode ray tube screen structure comprising a glasslike substrate containing a phosphor screen said screen having disposed thereon magnesium oxide strips made of crystalline magnesium oxide powder having a particle size the substantial portion of which is less than about 0.1 micron.

This invention relates to secondary-electron emissive materials for use in so-called beam-index type color picture tubes.

The primary object of the present invention is to provide magnesium oxide material improved for secondary electron emission in bean-index type color picture tubes.

There are other objects and particularities of the present invention, which will be made obvious from the follOWing description with reference to the accompanying drawings, in which:

FIG. 1a is a longitudinal sectional view of a so-called Apple tube, as one example of conventional beamindex type color picture tubes;

FIG. lb is an enlarged view of a portion of the glass panel shown in FIG. la;

FIG. 2 is a particle-size distribution diagram of magnesium oxide powder available in the market; and

FIG. 3 shows particle-size distribution of the material according to the present invention.

The so-called Apple tube is an example of conventional beam-index type color tubes. Referring to FIG. 1a, the Apple tube comprises a glass panel 1 with phosphor painted thereon. The phosphor screen 2 is a collectivity of vertical stripes painted on the inner surface of glass panel 1, and comprises red phosphor elements 3, blue phosphor elements 4 and green phosphor elements 5, deposited regularly on the panel 1. Aluminum is evaporated on the phosphor screen 2 to form an aluminum coating 7 for the purpose of improvement of brightness and protection against ion burning. Stripes 6 of magnesium oxide are deposited on the aluminum coating 7. There is a regularity between arrangements of phosphor stripes 3, 4, 5, and magnesium-oxide stripes 6. In Apple" tube, for example, one stripe of magnesium oxide is arranged in every three phosphor stripes. In FIG. 1b, a magnesium oxide stripe 6 is arranged in every two phosphor stripes.

When a phosphor screen of the above-described construction is scanned by an electron beam, the dilferences between secondary electron emissions from magnesium oxide stripes and aluminum face provide signals showing positions of electron beam. A signal of electron beam position thus obtained gives instruction which signal is to be selected among red, green and blue signals. Thus, this provides means for color lock.

In order to operate the beam-index type color picture tube in a stable and positive manner, the level of abovementioned signal should be sufiiciently higher than noise level. For this purpose, material of stripes capable of emitting secondary electron highly enough in comparison with the aluminum film surface is required. Magnesium oxide to be described below has heretofore been used as the stripe material, for the above-mentioned purpose. Further, such magnesium oxide stripes are requested to pass therethrough primary electrons with least deceleration or catch thereof as far as possible, and consequently, the thickness of stripe film should be less than 1 Magnesium oxide powder heretofore used for the secondary emission surface as above described is obtained by heating hydroxide, carbonate or nitrate of magnesium. Magnesium oxide powder thus obtained is spherical or aciculor in shape, and its particle size distribution is as shown in FIG. 2, particles above 0.1 micron particle size occupying the main part. This magensium oxide powder is deposited on the aluminum coating by polyvinyl alcohol (PVA) slurry method, preexposure method, etc. So-called PVA slurry method is to paint a mixture of aqueous solution of PVA, ammonium bichromate and magnesium oxide on the object, to harden the same by photochemical reaction, and to Wash out nonexposing part with pure water. The preexposure method is to deposit a mixture of aqueous solution of PVA and ammonium bichromate on the object, to dry the deposited mixture, to form hardened coating of PVA by photochemical reaction, flowing thereon a mixture of magnesium oxide and aqueous solution of PVA, and after drying, to develop the same with water. In such methods of deposition, the magnesium oxide is apt to become hydroxide by reaction with water, and therefore, the coating process of magnesium oxide should be effected as promptly as possible. In addition, magnesium oxide particles are so large that thin and even depositing thereof is difiicult to accomplish. Furthermore, the secondary electron emission, the most significant item in operation, is comparatively little, and sufiicient beam-index signal cannot be obtained. If the coating thickness of magnesium is increased in order to make signal output high, electron beam is correspondingly decelerated or caught thereby, thus lowering brightness of phosphor and deteriorating picture quality.

The present invention has succeeded to remove the above-described disadvantages of magnesium oxide for secondary emission. According to the invention, the material of secondary emission is crystalline magnesium oxide mainly consisting of particles of smaller than 0.1 micron. Such magnesium oxide may be obtained, for example, by the method to be described. Magnesium metal is first burnt in an atmosphere containing oxygen to obtain magnesium oxide. However, since magnesium oxide scatters in smoky state during combustion, magnesium metal is burnt in a closed vessel, and magnesium oxide deposited on the inner surface of the vessel is collected, or magnesium oxide smoke is taken into a suction device effectively, and by maintaining suitable oxygen pressure in the closed vessel during combustion, minute magnesium oxide powder is obtained. The particle size distribution of magnesium oxide powder thus obtained is exemplified in FIG. 3, from which it is clearly seen that the powder mainly consists of particles of size smaller than 0.1 micron. This magnesium oxide material is crystalline oxide of cubic system, and has reaction velocity with water very slow in comparison with other magnesium oxide materials. Magnesium oxide thus obtained is stable with respect to water in the afore-mentioned deposit method, and consequently, the deposit process is easy to effect, and even and very thin coating can readily be formed.

The secondary emission yield of such a magnesium oxide coating, that is, the ratio of secondary beam current to primary beam current, is about 2 at 20 kv. accelerating voltage of primary beam current, even when the coating thickness is as thin as 002 0.03 mg./cm. In comparison to the above, this value of conventional magnesium oxide material is about 1, and it is obvious that the magnesium oxide material of the present invention has very superior value. Experiments have shown that the use of such magnesium oxide material in screens of secondary electron emission of beam-index type color picture tubes provides enough signal output, and that, with the above-mentioned coating thickness, there is not seen substantial deceleration or capture of electrons passing through the magnesium oxide coating, with negligible deterioration of brightness of phosphor.

We claim:

1. A cathode ray tube screen structure comprising a glass-like substrate containing a phosphor screen, said screen having disposed thereon magnesium oxide strips made of crystalline magnesium oxide powder having a partial size the substantial portion of which is less than about 0.1 micron.

2. The tube of claim 1, wherein the phosphor screen comprises color phosphor elements containing an aluminum coating.

3. The tube of claim 1, wherein the thickness of the magnesium oxide strip is less than about 1 micron.

4. The tube of claim 1, wherein the thickness of the magnesium oxide strip is about 0.02 to 0.3 mg./cm.

5. The tube of claim 1, wherein the phosphor screen is a collection of substantially parallel strips disposed on the inner surface of the glass-like substrate and containing an outer conductive layer.

6. The tube of claim 5, wherein the color elements are alternating blue, red, and green strips.

7. The tube of claim 5, wherein the magnesium oxide strips are spaced apart so that one of said strips is provided for every two phosphor strips.

8. A beam-index type color picture tube comprising a glass substrate, phosphor strips disposed on the inner surface of said substrate, said phosphor strips being provided with an outer conductive layer, and magnesium oxide strips made of crystalline magnesium oxide powder having a particle size, the substantial portion of which is less than about 0.1 micron disposed on the aluminum coated surface, said magnesium oxide having a secondary beam current to primary beam current ratio of about 2 at 20 kv. accelerating voltage of the primary beam current.

9. The picture tube of claim 8, wherein the magnesium oxide strips have a thickness of about 0.02 to 0.03 mg./cm.

References Cited UNITED STATES PATENTS 3,054,672 9/1962 Angelucci 117-33.5 XR

WILLIAM L. JARVIS, Primary Examiner. 

